As with CCDs, cosmic ray hits will produce unwanted signal in the output images. However, no lasting damage to the detector pixels is expected from such hits. The NICMOS arrays have been subjected to radiation doses much higher than expected in their entire lifetime in accelerator tests without sustaining any long-term damage or measurable degradation in DQE. Hence, cosmic rays should have little impact on the long-term array performance in orbit.

The use of MULTIACCUM mode makes it possible to filter out cosmic rays since it provides a series of intermediate non-destructive reads, as well as the final image (see Chapter 8). These intermediate reads can be used to identify cosmic ray hits. The calibration pipeline, as described in the NICMOS Data Handbook, can identify and remove cosmic ray hits from MULTIACCUM observations.